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CAT's can hurt fuel economy when they become clogged. If it's legal, replace it with a new one. I found a Magnaflow Hi-Flow CAT for about $80 on ebay. I paid $40 to have an exhaust shop weld it in. I'm not concerned about the "hi flow" part, but it was 2-3 times cheaper than the OEM style CAT's, and it has worked well for almost 2 years now.
I suspect your O2 sensor could be going bad. Many sources I found explained that their usual lifespan is only 100k miles. Even if it wasn't the primary cause of your bad MPG, replacing it can be insurance for later on. After all, the O2 sensor helps determine how much fuel your motor needs to burn.
I would also do a complete tune-up of the engine. Before spending money on parts though, make sure to do a compression check for each cylinder. You want to make sure the engine is in good condition before blowing money on tuning it up. If the compression is high and even across all cylinders, do the full tune up. Plugs, wires, dist. cap/rotor, fuel filter, air filter, motor oil, transmission oil, etc. Following all that, make sure to check that the timing is right on the motor.
That would be a start. And remember, make sure your tires are pumped up to the correct pressure.
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